Low melting temperature compliant solders

ABSTRACT

Low melting temperature compliant solders are disclosed. In one particular exemplary embodiment, a low melting temperature compliant solder alloy comprises from about 91.5% to about 97.998% by weight tin, from about 0.001% to about 3.5% by weight silver, from about 0.0% to about 1.0% by weight copper, and from about 2.001% to about 4.0% by weight indium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication No. 60/720,039, filed Sep. 26, 2005, which is herebyincorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to solder compositions and,more particularly, to low melting temperature compliant solders.

BACKGROUND OF THE DISCLOSURE

As feature sizes of semiconductor devices continue to shrink, lowdielectric constant (low K) materials are more frequently employed toreplace conventional insulators (e.g., silicon oxide) in themanufacturing of semiconductor devices. Currently, carbon-doped siliconoxide (SiOC) (K˜2.5-3) is the industry's primary choice for a low Kmaterial in the manufacturing of semiconductor devices.

Carbon-doped silicon oxide (SiOC) typically comprises numerous airpockets to improve low K performance. However, these air pockets makethis low K material very brittle and susceptible to fracture.Consequently, during electronic packaging and assembly processes, thislow K material is known to crack due to stresses generated duringsoldering processes. In particular, solder paste reflow processesrequire reflow temperatures approximately 20-30° C. above the liquidustemperatures of solder alloys. For example, for a conventional Sn63Pb37solder paste, the reflow temperature is typically around 210-230° C.However, the recent conversion to Sn—Ag—Cu lead free solder alloys hasresulted in a great increase in reflow temperatures to typically around235-260° C. The liquidus temperatures and yield strengths of some ofthese Sn—Ag—Cu lead free solder alloys is summarized in the table ofFIG. 1.

Due to the higher liquidus temperatures (>218° C.) of the Sn—Ag—Cu leadfree solder alloys and mismatches in coefficients of thermal expansionbetween these Sn—Ag—Cu lead free solder alloys and low K materials, highstresses develop in low K materials during cooling from high temperaturereflow processes and thus cause cracking and failures in the low Kmaterials. In light of the above, solder alloys with lower meltingtemperatures are required.

In addition to the requirement for solder alloys with low liquidustemperatures, the ability of a solder to deform to accommodate possiblestresses or impact loading is critical to the reliability of electronicdevices employing low k materials. In general, solders with low yieldstrengths are softer and easier to deform so as to relieve stresses.Common low melting temperature solder alloys presently consist mainly ofgeneric 91Sn9Zn solder alloy and patented Sn—Ag—In and Sn—Ag—Cu—Insolder alloys. However, in comparison with Sn—Ag—Cu solder alloys, thesecommon low melting temperature solder alloys are at least 50% greater inyield strength and rigidity. A brief summary of these common low meltingtemperature solder alloys is provided in the table of FIG. 2.

As shown in FIG. 2, 91Sn9Zn solder has a melting point of 199° C., andthis solder is very strong (yield strength of 9.1 ksi) and very rigid.As also shown in FIG. 2, patented Sn—Ag—In and Sn—Ag—Cu—In solder alloysare also very strong and rigid. Specifically, U.S. Pat. No. 5,580,520discloses a solder alloy with (71.5-91.9)% Sn, (2.6-3.3)% Ag, and(4.8-25.9)% In, which has a melting point below 213° C., but is toostrong for use in low K material embedded semiconductor devices. Also,U.S. Pat. No. 6,176,947 discloses a solder alloy with (76-96)% Sn,(0.2-2.5)% Cu, (2.5-4.5)% Ag, and (6-12)% In, which has a liquidustemperature below 215° C., but has proven too rigid for use with low Kmaterial embedded semiconductor devices. Similarly, U.S. Pat. No.6,843,862 discloses an alloy composition with (88.5-93.5)% Sn,(3.5-4.5)% Ag, (2-6)% In, (0.3-1)% Cu, and up to 0.5% of an anti-oxidantand anti-skinning additive. This alloy is also too strong and rigid foruse in low K material embedded semiconductor devices. In addition, U.S.Pat. No. 6,689,488 reveals a solder alloy with (1-3.5)% Ag, (0.1-0.7)%Cu, (0.1-2)% In, balanced with Sn, but this alloy composition has shownto be either too high in melting temperature or too rigid for use in lowK material embedded semiconductor devices.

In view of the foregoing, it would be desirable to provide low meltingtemperature compliant solders which overcome the above-describedinadequacies and shortcomings.

SUMMARY OF THE DISCLOSURE

Low melting temperature compliant solders are disclosed. In oneparticular exemplary embodiment, a low melting temperature compliantsolder alloy comprises from about 91.5% to about 97.998% by weight tin,from about 0.001% to about 3.5% by weight silver, from about 0.0% toabout 1.0% by weight copper, and from about 2.001% to about 4.0% byweight indium.

In accordance with other aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy maycomprise at most about 3.0% by weight indium.

In accordance with further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy maycomprise at most about 2.5% by weight indium.

In accordance with still further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy mayfurther comprise traces of impurities.

In accordance with still further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy does notcomprise traces of impurities.

In accordance with additional aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy mayfurther comprise from about 0.01% to about 3.0% by weight at least onedopant selected from the group consisting of zinc (Zn), nickel (Ni),iron (Fe), cobalt (Co), germanium (Ge), phosphorus (P), aluminum (Al),antimony (Sb), cadmium (Cd), tellurium (Te), bismuth (Bi), platinum(Pt), rare earth elements, and combinations thereof to improve oxidationresistance and increase physical properties and thermal fatigueresistance.

In accordance with still additional aspects of this particular exemplaryembodiment, the rare earth elements may be selected from the groupconsisting of cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), lutetium (Lu), actinium (Ac), thorium (Th), protactinium(Pa), and combinations thereof.

In another particular exemplary embodiment, a low melting temperaturecompliant solder alloy comprises from about 89.7% to about 94.499% byweight tin, from about 3.5% to about 6.0% by weight silver, from about0.0% to about 0.3% by weight copper, and from about 2.001% to about 4.0%by weight indium.

In accordance with other aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy maycomprise at most about 3.0% by weight indium.

In accordance with further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy maycomprise at most about 2.5% by weight indium.

In accordance with still further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy mayfurther comprise traces of impurities.

In accordance with still further aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy does notcomprise traces of impurities.

In accordance with additional aspects of this particular exemplaryembodiment, the low melting temperature compliant solder alloy mayfurther comprise from about 0.01% to about 3.0% by weight at least onedopant selected from the group consisting of zinc (Zn), nickel (Ni),iron (Fe), cobalt (Co), germanium (Ge), phosphorus (P), aluminum (Al),antimony (Sb), cadmium (Cd), tellurium (Te), bismuth (Bi), platinum(Pt), rare earth elements, and combinations thereof to improve oxidationresistance and increase physical properties and thermal fatigueresistance.

In accordance with still additional aspects of this particular exemplaryembodiment, the rare earth elements may be selected from the groupconsisting of cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), lutetium (Lu), actinium (Ac), thorium (Th), protactinium(Pa), and combinations thereof.

The present disclosure will now be described in more detail withreference to exemplary embodiments thereof as shown in the accompanyingdrawings. While the present disclosure is described below with referenceto exemplary embodiments, it should be understood that the presentdisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein, and with respect to which the present disclosure maybe of significant utility.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present disclosure,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the present disclosure, but are intended to beexemplary only.

FIG. 1 is a table showing the liquidus temperatures and yield strengthsof several Sn—Ag—Cu lead free solder alloys.

FIG. 2 is a table showing the liquidus temperatures and yield strengthsof several common low melting temperature solder alloys.

FIG. 3 is a graph showing the effect of adding indium (In) to standardSn—Ag—Cu (SAC) alloys.

FIG. 4 is a table showing the liquidus temperatures and yield strengthsof indium (In) added Sn-1Ag-0.5Cu alloy compositions with respect to theconcentration of indium (In).

FIG. 5 is a table showing the liquidus temperatures and yield strengthsof indium (In) added Sn-2Ag-0.5Cu alloy compositions with respect to theconcentration of indium (In).

FIG. 6 is a table showing the liquidus temperatures and yield strengthsof indium (In) added Sn-2.5Ag-0.5Cu alloy compositions with respect tothe concentration of indium (In).

FIG. 7 is a table showing the liquidus temperatures and yield strengthsof indium (In) added Sn-3Ag-0.5Cu alloy compositions with respect to theconcentration of indium (In).

FIG. 8 is a table showing the liquidus temperatures and yield strengthsof indium (In) added Sn-4Ag-0.2Cu alloy compositions with respect to theconcentration of indium (In).

FIG. 9 is a graph showing the yield strengths of Sn—Ag—Cu—In alloys withrespect to the concentration of indium (In).

FIG. 10 shows a scanning electron microscopy (SEM) snapshot where energydispersive spectrometry (EDS) is used to identify major strengtheningparticles in an indium (In) added Sn—Ag—Cu alloy composition.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 3, there is shown a graph showing the effect of addingindium (In) to standard Sn—Ag—Cu (SAC) alloys. As shown in FIG. 3, theaddition of indium (In) to the standard Sn—Ag—Cu (SAC) alloys results ina decrease of liquidus temperature. Specifically, when indium (In) isadded to the standard Sn—Ag—Cu (SAC) alloys in an amount greater than2%, the liquidus temperatures of the resultant Sn—Ag—Cu—In alloys arereduced to below the liquidus temperatures of the standard Sn—Ag—Cu(SAC) alloys. Thus, it may be advantageous to utilize Sn—Ag—Cu—In alloyswith indium (In) concentrations greater than 2% in semiconductor devicesusing low K materials.

However, adding indium (In) to the standard Sn—Ag—Cu (SAC) alloys alsoresults in a rapid increase of the yield strength due to solutionhardening, and high strength Sn—Ag—Cu—In alloys may cause high stressesand unacceptable high defects. Thus, it would be beneficial to determinecompositional ranges for Sn—Ag—Cu—In alloys that result in low liquidustemperatures, low yield strength, and low rigidity. Indeed, the presentdisclosure is directed to Sn—Ag—Cu—In alloy compositions exhibiting lowliquidus temperatures, low yield strength, and low rigidity. SuchSn—Ag—Cu—In alloy compositions include Ag(0.001-3.5)%, Cu(0-1)%,In(2.001-4)%, balanced with Sn, and Ag(3.5-6)%, Cu(0-0.3)%,In(2.001-4)%, balanced with Sn. These Sn—Ag—Cu—In alloy compositionswere derived through a series of multiple experimentations asexemplified below.

EXAMPLE 1

The liquidus temperatures and yield strengths of indium (In) addedSn-1Ag-0.5Cu alloy compositions with respect to the concentration ofindium (In) are shown in the table of FIG. 4. The yield strengths of theresultant alloy compositions increased rapidly as the concentration ofindium (In) increased.

EXAMPLE 2

The liquidus temperatures and yield strengths of indium (In) addedSn-2Ag-0.5Cu alloy compositions with respect to the concentration ofindium (In) are shown in the table of FIG. 5.

The yield strengths of the resultant alloy compositions remained aboutconstant as the concentration of indium (In) increased up to 2.5%.However, when the concentration of indium (In) exceeded 2.5%, the yieldstrengths increased as the concentration of indium (In) increased.

EXAMPLE 3

The liquidus temperatures and yield strengths of indium (In) addedSn-2.5Ag-0.5Cu alloy compositions with respect to the concentration ofindium (In) are shown in the table of FIG. 6. The yield strengths of theresultant alloy compositions remained approximately constant as theconcentration of indium (In) increased up to about 2.5%. However, whenthe concentration of indium (In) exceeded 2.5%, the yield strengthsincreased as the concentration of indium (In) increased.

EXAMPLE 4

The liquidus temperatures and yield strengths of indium (In) addedSn-3Ag-0.5Cu alloy compositions with respect to the concentration ofindium (In) are shown in the table of FIG. 7. The yield strengths of theresultant alloy compositions decreased slightly as the concentration ofindium (In) increased up to about 2.5%. However, when the concentrationof indium (In) exceeded 2.5%, the yield strengths increased as theconcentration of indium (In) increased.

EXAMPLE 5

The liquidus temperatures and yield strengths of indium (In) addedSn-4Ag-0.2Cu alloy compositions with respect to the concentration ofindium (In) are shown in the table of FIG. 8. Due to a high yieldstrength (>6 ksi) developed because of a high silver (Ag) concentration(>3.5%), a lower copper (Cu) concentration (0.2%) with respect tostandard Sn—Ag—Cu (SAC) alloys (i.e., 0.5%) was employed. The yieldstrengths of the resultant alloy compositions decreased (approximately20%) as the concentration of indium (In) increased up to about 2.5%.However, when the concentration of indium (In) exceeded 2.5%, the yieldstrengths increased as the concentration of indium (In) increased.

The yield strengths of the Sn—Ag—Cu—In alloys with respect to theconcentration of indium (In) are shown in the graph of FIG. 9. As shownin FIG. 9, it is clear that the yield strengths of the indium (In) addedSn-1Ag-0.5Cu alloy compositions increased very rapidly as theconcentration of indium (In) increased, and thus these alloycompositions are unacceptable for use in low K material embeddedsemiconductor devices. However, with higher silver (Ag) concentrations,the yield strengths of the indium (In) added Sn—Ag—Cu alloy compositionseither remained about constant or decreased slightly as theconcentration of indium (In) increased up to about 2.5%, after which theyield strengths increased as the concentration of indium (In) increased.For example, the yield strengths of the indium (In) added Sn-2Ag-0.5Cu,Sn-2.5Ag-0.5Cu and Sn-3Ag-0.5Cu alloy compositions resulted in a slightdecrease in yield strength as the concentration of indium (In) increasedup to about 2.5-3%. However, as the silver (Ag) concentration increasedto 4% and the copper (Cu) concentration decreased to 0.2% (i.e.,Sn-4Ag-0.2Cu), the reduction in yield strength was very significant(approximately 20%), although this low yield strength compositionalrange was shortened very significantly. By the same token, it isreasonable to expect that as the silver (Ag) concentration becomesgreater than 4% (e.g., Sn-6Ag-0.2Cu), an even more significant reductionin yield strength would be produced, but the low yield strengthcompositional range would become even shorter. These results indicatethat the yield strengths of indium (In) added Sn-(0-2)% Ag-0.5Cu alloycompositions increase as the concentration of indium (In) increases, butthe yield strengths of indium (In) added Sn-(2-3.5)% Ag-0.5Cu alloycompositions decrease as the concentration of indium (In) increases(i.e., (2.001-4)% In). The latter alloy compositions give rise to thelow melting temperature compliant solders of the present disclosure foruse in low K material embedded semiconductor devices. In addition, whenthe copper (Cu) concentration is further reduced to 0.2%, the yieldstrengths of indium (In) added Sn-(3.5-6)% Ag-0.2Cu alloy compositionsare most significantly reduced.

In order to obtain a better understanding of the above results, scanningelectron microscopy (SEM) and energy dispersive spectrometry (EDS) wereperformed on the above mentioned alloys. For example, FIG. 10 shows anSEM snapshot where EDS is used to identify major strengthening particlesin an indium (In) added Sn—Ag—Cu alloy composition. As shown in FIG. 10,the major strengthening particles of this indium (In) added Sn—Ag—Cualloy composition is identified using EDS to be Sn_(66.6)Ag_(29.4)In₄.Specifically, the bright domains may be identified as Sn—Ag—In withinthe composition Sn_(66.6)Ag_(29.4)In₄, and the dark grey matrix may beidentified as a solid solution of indium (In) in tin (Sn). This is incontrast to the well established microstructure of the standard Sn—Ag—Cu(SAC) alloys where the major strengthening Ag₃Sn particles (the minorstrengthening particles are Cu₆Sn₅ due to copper (Cu)) are homogeneouslydistributed in the tine (Sn) matrix. That is, because of the addition ofindium (In) to the stoichiometric Ag₃Sn, the indium (In) dopedSn_(66.6)Ag_(29.4)In₄ particles are disordered and off-stoichiometric.More specifically, these off-stoichiometric Sn_(66.6)Ag_(29.4)In₄particles do not strengthen the solder as much as Ag₃Sn particles do dueto a softer nature of the off-stoichiometric compounds and a loss ofcoherency in the tin (Sn) matrix.

In addition, it has been discovered that solution hardening of indiumwas typically the main mechanism for strengthening Sn—Ag—Cu—In solderalloys. However, in the Sn—Ag—Cu—In compositions of the presentdisclosure, indium (In) is removed from the solution, thus reducing thesolution hardening effect, and instead forms the off-stoichiometricSn_(66.6)Ag_(29.4)In₄ particles, which did not strengthen the alloy asmuch as the replaced stoichiometric Ag₃Sn particles. As a result of theabove-mentioned effects, the yield strengths of the presently disclosedindium (In) added Sn—Ag—Cu alloy compositions decrease as theconcentration of indium (In) increases (i.e., between (2.001-4)% In).

FIG. 10 also reveals that as the concentration of silver (Ag) decreasesbelow 2%, Sn_(66.6)Ag_(29.4)In₄ particles are found to be sparselydistributed because less indium (In) is removed from the solution, andthe softening effect is negligible. In contrast, as the concentration ofsilver (Ag) exceeds 6%, indium (In) available to formSn_(66.6)Ag_(29.4)In₄ particles is exhausted. Nevertheless, the numberof Ag₃Sn particles continues to increase due to the increasing amount ofavailable silver (Ag), rendering the softening effect less conspicuousand the low strength compositional range shorter. In accordance with thepresent disclosure, further reduction of yield strength is achieved byreducing the number of the minor strengthening particles of Cu₆Sn₅ byreducing the copper (Cu) concentration, thereby resulting in even moreadvantageous alloy compositions.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Further, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

1. A low melting temperature compliant solder alloy consistingessentially of from about 91.5% to about 97.998% by weight tin, fromabout 0.001% to about 3.5% by weight silver, from about 0.0% to about1.0% by weight copper, and from about 2.001% to about 4.0% by weightindium.
 2. The low melting temperature compliant solder alloy of claim1, wherein the alloy comprises at most about 3.0% by weight indium. 3.The low melting temperature compliant solder alloy of claim 1, whereinthe alloy comprises at most about 2.5% by weight indium.
 4. The lowmelting temperature compliant solder alloy of claim 1, wherein the alloyincludes traces of impurities.
 5. The low melting temperature compliantsolder alloy of claim 1, wherein the alloy does not include traces ofimpurities.
 6. The low melting temperature compliant solder alloy ofclaim 1, further consisting of from about 0.01% to about 3.0% by weightat least one dopant selected from the group consisting of zinc (Zn),nickel (Ni), iron (Fe), cobalt (Co), germanium (Ge), phosphorus (P),aluminum (Al), antimony (Sb), cadmium (Cd), tellurium (Te), bismuth(Bi), platinum (Pt), rare earth elements, and combinations thereof toimprove oxidation resistance and increase physical properties andthermal fatigue resistance.
 7. The low melting temperature compliantsolder alloy of claim 6, wherein the rare earth elements are selectedfrom the group consisting of cerium (Ce), lanthanum (La), praseodymium(Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), actinium (Ac),thorium (Th), protactinium (Pa), and combinations thereof.
 8. A lowmelting temperature compliant solder alloy consisting essentially offrom about 89.7% to about 94.499% by weight tin, from about 3.5% toabout 6.0% by weight silver, from about 0.0% to about 0.3% by weightcopper, and from about 2.001% to about 4.0% by weight indium.
 9. The lowmelting temperature compliant solder alloy of claim 8, wherein the alloycomprises at most about 3.0% by weight indium.
 10. The low meltingtemperature compliant solder alloy of claim 8, wherein the alloycomprises at most about 2.5% by weight indium.
 11. The low meltingtemperature compliant solder alloy of claim 8, wherein the alloyincludes traces of impurities.
 12. The low melting temperature compliantsolder alloy of claim 8, wherein the alloy does not include traces ofimpurities.
 13. The low melting temperature compliant solder alloy ofclaim 8, further consisting of from about 0.01% to about 3.0% by weightat least one dopant selected from the group consisting of zinc (Zn),nickel (Ni), iron (Fe), cobalt (Co), germanium (Ge), phosphorus (P),aluminum (Al), antimony (Sb), cadmium (Cd), tellurium (Te), bismuth(Bi), platinum (Pt), rare earth elements, and combinations thereof toimprove oxidation resistance and increase physical properties andthermal fatigue resistance.
 14. The low melting temperature compliantsolder alloy of claim 13, wherein the rare earth elements are selectedfrom the group consisting of cerium (Ce), lanthanum (La), praseodymium(Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), actinium (Ac),thorium (Th), protactinium (Pa), and combinations thereof.